1. Field of the Invention
The present invention relates to a separator for a nonaqueous electrolyte secondary battery and a battery including the same. More particularly, the present invention relates to a separator which includes a resin-made substrate and a porous heat resistance layer disposed on the substrate.
2. Description of Related Art
A nonaqueous electrolyte secondary battery such as a lithium-ion secondary battery, a nickel-hydrogen battery or the like is extensively used as a so-called portable power supply or as a power supply for driving a motor vehicle. In particular, a lithium-ion secondary battery, which is light in weight and which is capable of obtaining a high energy density, is desirably used as a high-output power supply for driving a motor vehicle such as an electric vehicle, a hybrid vehicle or the like. In general, the nonaqueous electrolyte secondary battery of this kind includes a positive electrode, a negative electrode and a separator. The separator is typically formed of a resin-made porous sheet. The separator has a function of electrically insulating the positive electrode and the negative electrode, a function of holding a nonaqueous electrolyte, and a shutdown function. Further, a predetermined level of heat resistance (durability) is required in the separator. In other words; even when thermal contraction or fracture occurs in the separator due to overheating or other causes, it is required for the separator to prevent occurrence of an internal short-circuit of the positive electrode and the negative electrode, thereby suppressing a temperature rise in the battery as a whole.
As means for responding to such a requirement, there is proposed a separator which includes a porous heat resistance layer (HRL) formed on a surface of a resin-made porous sheet. Typically, the porous heat resistance layer is mainly composed of inorganic compound particles (inorganic filler) and is high in heat resistance and insulation property (non-conductive property). For example, Japanese Patent Application Publication. No. 2013-149434 (JP 2013-149434 A) discloses a separator for a nonaqueous electrolyte secondary battery in which two kinds of insulating inorganic fillers are contained in a porous' heat resistance layer.
However, in the aforementioned battery, there is a possibility that the cycle characteristic is deteriorated or the operation of a current interrupt device (CID) when overcharged is delayed. The inventors have studied this phenomenon from various viewpoints and have found that a stress that may be applied to the separator is involved. In the application such as a vehicle-driving power supply or the like, for the purpose of realizing a high capacity, the battery is generally used in the form of a battery pack fabricated by arranging and confining a plurality of batteries. At this time, a higher stress may be applied to the porous separator than the stress applied to other members. When the battery is charged and discharged (when an active material occludes and emits charge carriers), an active material layer repeats expansion and contraction. Therefore, there may be a case where the separator adjoining the active material layer is gradually crushed and made thin. In this case, the separator's function of holding a nonaqueous electrolyte (liquid retention characteristic) may be impaired. The distance between the positive electrode and the negative electrode may be locally reduced, thereby generating a micro short-circuit, or the reaction of a gas generating agent when overcharged may be inhibited. Accordingly, in order to improve the cycle characteristic or to enhance the operability of the current interrupt device when overcharged, it is necessary to provide a separator which is superior in shape-keeping characteristic (shape stability).